This article originally appeared in the March 98 Newsletter of the Liverpool Branch

And still more on the Role of Nitrogen

by Ray Allcock.

The present article comprises a few somewhat disconnected observations and practical hints arising in further consideration of the feed requirements of cacti and succulents, as already discussed in my article in the January 1998 Branch Newsletter. It should therefore be read as a supplement to the latter.

1. As mentioned in the January article, some nitrogen compounds are chemically rather unstable in the presence of water, and gradually decompose. Ammonium nitrate is one such, and moreover both it and of course any compound fertilizer containing it strongly attract water vapour and become very wet. To avoid this it is best, once the polythene packet is breached, to use screw-top jars having rubberized seals in the lids. I find that the papery seals in the lids of instant coffee jars are entirely incapable of stopping the ingress of moisture!

2. I have observed that when well-fed plants are deprived of feed they continue to grow for several months, evidently therefore living on their reserves. Only thereafter do the manifestations of malnutrition (described in the Branch Newsletter of January 1995) start to become apparent. One of the consequences of malnutrition is that the plants, although perhaps still looking quite healthy, lose the capacity to make new roots or even, in serious cases, to keep the old ones. In view of this I now consider it to be unwise, when a new plant is acquired, to transfer it straight away into one’s own favoured compost. Better by far to take it as it is and to feed it regularly and well until it is seen to be in a vigorous growing condition.

3. For the same reasons as given under 2 above one should in one’s own repotting give priority to plants which are obviously fit and healthy, and meanwhile to try to nurse back to health by careful feeding and watering any which appear to be malnourished. In this context it is of course needful to ascertain the cause of the malnourishment — plants can with care be reinvigorated even in poorly structured or compacted composts, but it doesn’t help to leave them in such composts if the roots are already dead!

4. When a chronically starved plant is put onto an adequate feeding regime (for which see the January 1998 article) it may still take one or two years, or sometimes even more, before a completely normal growth becomes reestablished. There will usually be an intermediate period of adjustment, in which the new growth is likely to be geometrically irregular and poorly spined. The onset of this poor sort of growth indicates that with continued correct feeding the plant will most likely make in the end a full and rewarding recovery!

5. Within my collection there are quite a lot of plants which, from enforced neglect and lack of understanding, have been left to languish for ten or twenty years or more without repotting. These plants have for most of the time made only a small amount of growth. They are sitting in compacted composts made principally of loam and sand; in a few cases peat and sand and loam. Miraculously, these composts, although now looking close and dusty, are still free-draining (i.e. water sinks rapidly). During all those long years they were given a regular but very meagre feed, as described in the January 1998 article. I used to think that soil compaction was to blame for their lack of growth, but in that I was mistaken. The heavy feeding of the last two years has led in most cases to a resumption of steady growth, albeit not so abundant as that which I get with better-aerated composts. The clear implication here is that adequate nutrition is the sine qua non for growth, and that while plentiful aeration further enhances the growth it is nevertheless not the primary factor.

Another factor relevant in regard to these neglected plants is that the loam was in many cases quite alkaline, due presumably to my having an old and over-limed kitchen garden, in which the pH hovers about the value 7.5. Since early 1993 I have been allowing for this by adding about 1 level teaspoon of the acidifying agent Aluminium Sulphate to every gallon of water, and over the years this has brought the pH down to 6.5 or a little less. From the very inception of this treatment most of the plants which still had live roots showed an instantaneous improvement (well, almost, over one or two days actually), but it was only with the heavy feeding that the newly established state of growth accelerated to the present relatively satisfactory level.

6. About 8 years ago I repotted a large number of my cacti into a new compost based upon an organically slightly enriched loam and a lot of crushed brick, and having excellent properties of aeration and absorbency (see the Branch Newsletter of March 1996). Initial results struck me at that time as fantastic, but alas, the organic nutrient content was in actuality very small and, much worse than that, the alkalinity within the brick fragments soon leaked out and came to dominate the humic acids present. The growth was excellent in the first year but came to a dead stop in the second year. It was then found that the pH, originally 7, had risen to 8. This problem has now also been rectified with the help of aluminium sulphate, as under 5 above, and consequential to the heavy feeding of the last two years many of these plants are now ready for potting-on.

In view of the extreme nature of this particular case of wrong pH I did in fact give several waterings at 25ml of aluminium sulphate per gallon, with no adverse consequences whatever. The mentioned chemical is in fact very gentle yet powerful in its action, and will not lead to aluminium poisoning provided the final pH of the compost is higher than 5. It can be bought in small amounts under the designation ‘Hydrangea Colourant’, and is doubtless also an ingredient in the so-called acid feeds.

7. There are two sorts of potting operation. In the so-called potting-on one replants the whole rootball intact. In the so-called repotting one removes the old compost from the roots, and then replants. In my experience the Mesembs do not dislike repotting, since they grow new feeder roots every year anyway. But other succulents and the cacti find the operation more traumatic, and therefore in cases where the state of the compost makes its removal necessary it is wise not to be too eager to remove every stubborn scrap. One has to balance the aim of the operation - namely improved aeration etc. - against the possibility that an excess of misdirected zeal may lead to too great a shock or to small but insidiously lethal wounds. Potting on may be undertaken at any time of the year but repotting should be done only when the ambient temperature stays above 55ºF (13ºC). The repotted plants should be put in a shady but warm location and given a light spraying several times per week. They should not be watered until the presence of new growth indicates that the roots have reestablished themselves in the new compost. Prior to that the spraying will already suffice to keep the compost slightly damp. It is also beneficial to have it very slightly damp at the time of repotting, since this will ensure a good capillary action (i.e. will ensure that the compost is subsequently ‘wettable’).

Since effective liquid feeds and acidifiers are readily available, the tedious and time-consuming process of repotting will usually be judged to be necessary only on account of a compacted and impenetrable root ball; a root ball which, if left as it is, will in various ways deprive the plant of air and water. When such a root ball is freed up roots of a quite excessive length are often revealed. These long roots have obviously been vainly looking for fresh supplies of food and drink, which of course are not available within the cramped space of a pot. Their great length necessitates a long journey for any food and drink picked up, since it is only the root tips which are active in this respect, and the plant is in consequence put at a disadvantage. Corrective action is called for, i.e. root surgery. The long roots should be cut back to lengths commensurate with the size of the intended pot. Sometimes one is faced with a tangle of roots so densely overgrown and intertwined that it seems impossible even to begin the operation of removing or loosening the old compost. It may not even be anywhere exposed to view! In such an eventuality the only practical procedure is simply to cut the tangle right off, taking care not to damage the thicker roots. When the latter are finally exposed and freed up it is well also to cut them back somewhat, as just explained. Then all cut ends should be dusted with powdered sulphur or other powdered antiseptic, and the plant left for a few days in a warm shady location while the cut ends heal over. It can then be potted up and treated as recommended above for repotted plants. Plants treated surgically as described often respond with great vigour as the new short feeder roots grow out into the new compost.